1. Field of the Invention
The present invention relates to an electric motor control device for vector control of an induction motor, more particularly relates to an electric motor control device able to speed the rise of magnetic flux and issue an excitation current instruction preventing overshoot of magnetic flux.
2. Description of the Related Art
An induction motor runs a primary current through a stator to generate a rotating magnetic field and has the magnetic flux caused by the rotating magnetic field cut across by a rotor so as to induce voltage at the rotor and cause the flow of a secondary current. It uses the interaction between this secondary current and the magnetic flux to generate torque. In the past, as the control of the induction motor, vector control dividing the primary current flowing through the stator into an excitation current of the magnetic flux direction and a secondary current, that is, a torque current, has been used. The torque generated is proportional to the product of magnetic flux generated by the excitation current and torque current.
FIG. 9 is a view explaining vector control of a conventional induction motor. A torque instruction instructing the torque of the induction motor is input to a current controller 5 and is input to the magnetic flux instruction processor 4 for output of a magnetic flux instruction. The magnetic flux instruction is processed considering the rotational speed detected by a speed sensor 71 of an induction motor 7 in the magnetic flux instruction processor 4. The magnetic flux instruction is input in the current controller 5 in the same way as the torque instruction.
The torque instruction is input to a torque current processor 53 of the current controller 5. The torque current processor 53 processes the torque (q-phase) current instruction. The difference between the output torque current instruction and the torque actual current fed back from the voltage conversion device 6 is input to a torque current controller 54. The torque current controller 54 processes and outputs a d-phase voltage instruction to be input to a dq-uvw converter 56 in accordance with the difference of the input torque current.
On the other hand, the excitation instruction is input to an excitation current processor 51 of the current controller 5. The excitation current processor 51 outputs an excitation (d-phase) current instruction. The difference between the output excitation current instruction and the excitation actual current fed back from the voltage conversion device 6 is input to an excitation current controller 52. The excitation current controller 52 processes and outputs the q-phase voltage instruction to be input to the dq-uvw converter 56 in accordance with the difference of the input excitation current.
The dq-uvw converter 56 converts the input d-phase voltage instruction and q-phase voltage instruction to a u-phase voltage instruction, v-phase voltage instruction, and w-phase voltage instruction. The u-phase voltage instruction, v-phase voltage instruction, and w-phase voltage instruction are input to the voltage conversion device 6 as output of the current controller 5. The u-phase voltage instruction, v-phase voltage instruction, and w-phase voltage instruction input to the voltage conversion device 6 are converted by the voltage conversion device 6 to the actual currents of the uvw phases supplied to induction motor 7. The induction motor 7 is driven by the actual currents of the uvw phases. The induction motor 7 outputs the torque instructed by the torque instruction and makes the shaft 8 rotate.
Note that the actual currents of the uvw phases output from the voltage conversion device 6 are fed back to the current controller 5 where the uvw-dq converter 55 of the current controller 5 converts them to the torque (d-phase) real current and excitation (q-phase) real current which are used as the torque current feedback and excitation current feedback. Further, the speed sensor 71 of the induction motor 7 feeds back the rotational speed of the induction motor 7 to the magnetic flux instruction processor 4.
When raising the magnetic flux of such an induction motor, when changing the magnetic flux instruction for improving the response of control, etc., it is sought to quickly raise or change the magnetic flux. However, the magnetic flux generated rises by a time constant determined from a circuit constant with respect to the excitation current, so until the magnetic flux is established, it becomes delayed as compared with the excitation current.
Therefore, in the past, to shorten the rising time of magnetic flux, the practice had been to multiply the magnetic flux instruction with a certain boost coefficient to give a value larger than the instruction value for generating magnetic flux from the time of rise of the magnetic flux to when the estimation value of the magnetic flux reaches the instruction value. However, as a result of multiplication of the boost coefficient, the generated magnetic flux always overshoots. There was therefore the problem that the time until the magnetic flux instruction value was reached became extended (see Japanese Patent Publication (B2) No. 6-67253).